The next-generation electrical grid will use advanced software, monitoring and renewable power sources to make the North American electrical supply more robust. But for maximum flexibility during peak demand, the grid also must be able to store power and supply it on demand.

With government mandates such as the California Public Utilities Commission’s 2013 rule requiring investor-owned utilities to procure 1.325 gigawatts of cost-effective energy storage by 2020, the push for a storage solution becomes more urgent.

Flow batteries are emerging as a leading option in long-duration industrial energy storage applications. In essence, they allow intermittent energy resources to be regulated from moment to moment, says Bill Radvak, CEO of integrated energy storage company American Vanadium in Vancouver, British Columbia, Canada. This “dispatchable power” better enables the grid to balance the amount of energy placed on the grid as demand rises and falls.

Although different types of batteries have been researched and tested for long-duration energy storage, flow batteries are starting to be installed at the demonstration level on the grid. They also show a promising cost trajectory, as flow battery manufacturers "think they can make significant cost reductions given greater production scale," says Andy Lubershane, senior analyst at IHS Emerging Energy Research.

In a flow battery, chemical solutions serve as the positive and negative electrolytes. They are typically stored in separate tanks and delivered to an electrochemical cell stack (resembling a fuel cell) via a recirculation pump. Some designs eliminate one of the tanks by adding passages within the remaining container to separate and recirculate the electrolyte to the electrodes.

“By recirculating the electrolyte through the cell stack from the central reservoir, we can achieve better control over the electrochemical process in the cells than you can in battery types where the electrolyte is static,” says Jonathan Hall, vice president of engineering for flow battery manufacturer Primus Power in Hayward, Calif.

Scalability

Flow batteries are scalable in that the cell stack size can be increased if more power is needed for an application. Adding more electrolyte results in a battery that runs longer. “That is a key benefit of flow batteries,” says Brad Fiebig, who leads business development for Lockheed Martin’s energy storage business. “With a flow battery, you have the flexibility to only pay for more run time (energy) if that is all you want. With a conventional sealed battery, you also have to pay for more power even if all you wanted was more run time.” (Lockheed Martin acquired Sun Catalytix Corp., an energy storage technology developer specializing in flow battery technology, in August 2014.)

Flow battery technology will compete with today’s energy storage technologies as well as other advanced technologies including sodium sulphur, sodium nickel chloride and flywheels for a share of the grid-connected energy storage market, according to a report from IHS Technology. Although only 200 MW of energy storage systems currently are installed in the U.S., annual installations are expected to reach more than 2,500 MW in 2017.

Leading the Charge

“If you look at the biggest issue with the energy grid in North America, it’s not the expanding use of energy , it's the peak consumption that continues to increase,” Radvak says. “Utilities are required to have 100% reliability on the hottest days of the summer. To do that historically involves bringing more generation, more substations and more infrastructure.”

Flow batteries present an alternative as they can store up to 12 hours of charge at a time as the electrolyte circulates for constant recharging. Their lifespan also is measured in decades. By comparison, a lithium-ion battery has a lifespan of 5-7 years.

“Once you get to industrial power, lithium batteries lose their effectiveness as a solution both in cost and technical abilities,” Radvak says. “Every time you charge or discharge one of those batteries, you are degrading their efficiency.”

American Vanadium’s flow battery, called CellCube, uses vanadium as its electrolyte. Vandium is the only element that enables one element on both the anode and the cathode, says Radvak. The result is little if any electrode degradation "so you really keep that same efficiency level for 20 years,” he says. (In late October, the company said its CellCube had received Underwriters Laboratories 508A certification.)

The Metropolitan Transportation Authority and New York City Transit Office of Strategic Innovation and Technology are testing that performance with a CellCube system in Manhattan. The energy storage system will be used for demand reduction: literally taking power off the grid at night when it is less expensive and putting it into the building’s energy system at peak demand periods.

Another factor driving grid-connected energy storage is the push by states to use increasing amounts of renewable resources. The U.S. Energy Information Administration (EIA) projects that wind power capacity will increase by 16.2% in 2015, contributing 4.7% of total electricity generation capacity. Utility-scale solar energy will represent 0.6% of energy generation capacity in the U.S., although the EIA says that most growth in this sector historically comes from customer-sited distributed solar. Since neither solar nor wind is dispatchable, energy storage is necessary in order to maintain a stable, reliable grid.

Road to Viability

To become commercially viable, flow batteries must prove their worth in several areas. Utilities first need to see results before investing in the technology. A growing number of installations and demonstrations aim to provide that information. The U.S. Department of Energy’s National Renewable Energy Laboratory has commissioned American Vanadium’s CellCube for its megawatt-scale Energy Systems Integration Facility to test for renewable-integration and utility-scale applications.

Primus Power is supplying its EnergyPod zinc-flow batteries for three projects. A 280 kW/1 MWh unit will integrate with an existing 260 kW photovoltaic system to form a microgrid that will reduce peak electrical demand and provide power to serve critical functions should a grid fail. On Washington State’s Bainbridge Island in Puget Sound, meanwhile, two EnergyPods will support a research and demonstration project targeting power reliability on the island and increasing local capacity. For the third project, Modesto Irrigation District in central California has purchased 25 MW of energy storage primarily to balance a natural gas-fired peaking plant and renewable energy resources.

Flow batteries offer the promise of cost effectiveness, but affordability poses a second concern in the early stages of the technology’s commercial deployment.

“Although there is growing demand for electricity to be cleaner and more reliable, electricity is a commodity that we all expect to be provided as cost-effectively as possible,” says Lockheed Martin’s Fiebig. “The infrastructure that generates and manages that commodity obviously has to meet that affordability expectation. Energy storage typically has not met that expectation, [which is why] you haven’t seen widespread adoption.”
He says that lowering the cost of energy storage is not just a matter of scaling up production volume or going to a low-cost labor market to meet a price point. "In the case of electrochemical energy storage, there is a fundamental engineering challenge to meet affordability.”

That engineering challenge, Fiebig says, is developing the right chemistry and production processes that optimize the durability and safety of the overall product, therefore enabling widespread adoption of energy storage.

In 2013, MIT researchers tried to address the expense concern when they engineered a flow battery prototype that eliminates the costly membrane. The system reportedly generates three times as much power per square centimeter as other membrane-less designs. The team estimates that such a flow battery could enable an energy cost of $100 per kilowatt hour (kWh). IHS analyst Lubershane says that $100/kWh of storage is the necessary price point “before batteries can compete with gas combustion turbines to provide the peaking resource that the grid requires.” The U.S. Energy Department's Advanced Research Projects Agency-Energy (ARPA-E) is targeting this same number as it supports research into grid-scale renewable energy storage technologies.

Service Life Issues

Tied to cost is the service life of the technology. Utility and industry customers have high expectations for the cycle life and maintenance of this type of equipment, Primus Power’s Hall says. “They would prefer to deploy a battery into their distribution networks that is as long-lived and low maintenance as something like a transformer, which gets installed once and is left alone for a long time.”

Flow battery density also poses problems. Simply stated, they take up much more space to hold the same amount of energy compared with alternatives, notably lithium-ion batteries.

"That’s important because the greatest near-term opportunity for batteries is likely to be in dense, urban areas where space is at a premium," Lubershane says.

Despite some of the hills that flow batteries still must scale, recent improvements in cost and efficiency, alongside new regulatory requirements, show that electrical storage will be a big part of the next-generation power grids that take full advantage of renewable power sources.

More Resources:

IHS Energy Storage Intelligence Service